The subunit structure of the Drosophila melanogaster embryo DNA polymerase will be determined on highly purified preparations isolated from 12-18 hr old embryos. The structure of these preparations will be compared with those of the enzymes obtained at different developmental stages during embryogenesis, and with the structures generated by in vitro trypsin treatment of highly purified DNA polymerase. Pyridoxal phosphate will be used as a probe for the deoxynucleotide-binding subunit of various forms of the DNA polymerase. By comparing the results from the different size classes of DNA polymerase (from 5.5S, generated by trypsin, to 9.0S) it will possible to demonstrate the nature of the deoxynucleotide-binding subunit during the different stages of enzyme conversion. Apparent activation of the 9.0S Drosophila DNA polymerase by proteolytic cleavage will be studied to determine whether the site of attack is, indeed, the largest subunit and whether cleavage generates catalytic subunits or inactivates a modulatory subunit. The time and site of synthesis of the embryonic DNA polymerase will be determined by examining radio-labelled developing oocytes for the presence of DNA polymerase subunits.